The use of metallocene compounds in polymerization catalysts, and the use of metallocene catalysts for polymerization are known. However, there remains an ongoing effort to develop metallocene catalysts, polymerization processes using such catalysts, and polyolefin resins and products made therewith, each having advantageous properties and performance.
There are numerous references discussing metallocene catalyst systems comprising at least two catalyst components, wherein at least one component is a metallocene catalyst. For example, U.S. Pat. No. 4,530,914 discusses a catalyst system for producing polyethylene having a broad molecular weight distribution using two different metallocene-type catalyst compounds having different propagation and termination rate constants for ethylene polymerization. U.S. Pat. No. 4,937,299 is directed to a homogeneous catalyst system of at least two metallocene-type catalyst compounds each having different reactivity ratios for use in a single reactor to produce a polymer blend. U.S. Pat. No. 5,470,811 discusses producing polymers having improved product properties using an isomeric mixture of two or more substituted metallocene-type catalyst compounds. EP-A2-0 743 327 discuss the use of meso and racemic bridged mixed metallocene-type catalysts compounds to produce ethylene polymers having improved processability. U.S. Pat. No. 5,516,848 relates to a process for producing polypropylene using a bridged mono-cyclopentadienyl heteroatom containing compound and an unbridged, bis-cyclopentadienyl containing compound. EP-B1-0 310 734 discusses the use of a mixed bridged hafnium and zirconium metallocene-type catalyst compounds to produce a polymer having broad molecular weight distribution. U.S. Pat. No. 5,696,045 describes using at least two different bridged zirconium metallocene-type catalyst compounds to produce propylene polymers having a broad molecular weight distribution where one of the stereorigid zirconocenes has an indenyl ligand having a substituent on the six-member ring. EP-B1-516 018 describes using two different bridged zirconium metallocene-type catalyst compounds to produce a broad molecular weight distribution polypropylene polymer where one of the bridged metallocenes has indenyl ligands that are substituted in at least the two position. U.S. Pat. No. 6,492,472 discusses a catalyst system comprising at least two different metallocene compounds, the system comprising a bridged metallocene catalyst compound and a bridged, asymmetrically substituted, metallocene catalyst compound useful for producing polymers generally having a narrow molecular weight distribution. U.S. Pat. No. 7,141,632 discusses a mixed metallocene catalyst system including a poor comonomer incorporating metallocene catalyst compound and a good comonomer incorporating metallocene catalyst compound useful in producing a more broadly separated bimodal polymer composition. U.S. Pat. No. 7,163,906 discusses a catalyst composition comprising the contact product of at least one metallocene compound and at least one organochromium compound. U.S. Pat. No. 7,172,987 discusses a bimetallic catalyst system comprising a modified Ziegler-Natta catalyst component and a metallocene catalyst component useful for producing bimodal polymers, specifically polyethylene, having a polydispersity of from 12 to 50.
Additionally, there are also references directed to polymerization processes in which two or more polymerization reactors are joined in series, where one catalyst is used in a first reactor to produce a first polymer that is then fed into a second reactor with the same or different catalyst, typically under different reactor conditions. In this way, the resulting polymer from a series polymerization process is a combination or blend of two different polymers. These polymer blends, typically, contain a high molecular weight and a low molecular weight component. For example, U.S. Pat. No. 5,665,818 discusses using two fluidized gas phase reactors in series using a transition metal based catalyst to form an in situ polymer blend having improved extrudability. EP-B 1-0 527 221 discusses a series reactor process using metallocene-type catalyst systems for producing bimodal molecular weight distribution polymer products. However, series or multistage reactor processes are expensive and more difficult to operate.
In addition, a polymer produced in a single reactor having a plurality of desirable properties and/or characteristics remains elusive. This is especially true in the area of films. Films are often a tradeoff of properties. A film having a combination of good optical properties, e.g., clarity, haze, and the like; in combination with a film having improved dart impact strength and/or high melt strength would be of particular interest for use in various end-use applications. However, such a film, which may be produced on a commercial scale, remains elusive in the art.
Accordingly, there remains a need for catalyst systems with good reactor operability which is capable of producing a polymeric film having improved optical properties in combination with improved dart impact strength and/or high melt strength as compared to films known in the art. There is also a need for olefin polymers and polyolefin products having these enhanced properties. These and other limitations may be overcome with the catalysts, methods, polymers, and products of the present disclosure.